I don’t really blame ECS, my rear bumper was a bit tired from removal and reinstall of my stock diffuser for a wrap attempt and than a few Plastidip re-sprays.
I was able to address the fitment issues by building up the corner and lower tabs using JB Weld.

I built up the offending tabs, but retained their wedge shape and depth. This enables the tabs to still slide into position and remain locked in place. The carbon fiber does not flex like the OEM plastic part...so it requires additional tension to remain in place.
When reinstalling, I used some clamps to hold the corners while I set the lower tabs and screws.

No more gaps...

I had daylight on this side...

I hope this helps those with fitment issues due to tired plastic bumpers.
I would advise ECS to increase thickness on the lower and corner tabs on future carbon fiber products. The lack of flex in carbon fiber requires that the part really grip the bumper. Sometimes the plastic on these bumpers is a bit worn and unable hold the rigid carbon without some help.
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